142 



THE DEVELOPMENT OF THE CHICK 



inevitably be thrown into secondary folds by the approximation 

 of points c and d (Fig. 78 C). 



Study of the fusion of the amniotic folds in actual sections 

 shows, that the line of fusion of the opposite amniotic limbs is 

 over the dorsal surface of the embryo only so long as the latter 

 lies flat on the yolk; it does not follow the turning of the embryo 

 on to its left side, and the consequence is that, after rotation of 

 the embryo, the line of fusion lies over the upper (right) side of 

 the embryo, often opposite the horizontal level of the intestine 

 (Fig. 79). Thus one fold of the amnion passes all the way from 

 the under side over the back of the embryo and around on the 

 other side to the line of fusion, and thus is several times as long 

 as the opposite limb. Moreover, the amniotic fold of the right 



Fig. 79. — Section of an embryo of about 60 hours to show the sec- 

 ondary fold (s. f .) of the amnion on the right side. 

 e. a., Ectamnion. s. f., Secondary fold. 1., Left. r. 



Right. 



side is invariably thicker than that of the left side, and is always 

 thrown into secondary folds at the place of turning (Fig. 79). 

 These conditions are satisfactorily explained, as noted above, by 

 the mere turning of the embryo on its side. 



One must therefore distinguish in the upper limb of the am- 

 nion two kinds of folds: (1) The ordinary amniotic fold induced 

 by the fusion of the right and left folds, and (2) secondary folds 

 formed simply by the process of twisting of the embryo. 



These secondary folds of the amnion are very transitory, 

 except in two regions: (1) Above the hind end of the heart (apex 

 of ventricle), and continuing a short distance behind it; (2) in 

 the region immediately in front of the allantois, at sixty to seventy 

 hours, thus in the neighborhood of the final closure of the amniotic 



